Yawning can cause watery eyes, according to Healthline. When you yawn, you "scrunch" the muscles near your eyes, creating pressure on the lacrimal glands. The lacrimal glands — aka tear glands — are located just above the eye and below the outside corner of your eyebrow. They're tasked with producing the fluid that becomes tears when you blink, yawn, or cry, according to the Mayo Clinic.

Besides yawning, another symptom of tiredness is dry eyes, says Dr. Kanakamedala of Charlotte Eye, Ear, Nose & Throat Associates. To combat dry eyes, your body creates "basal tears" that are made up of "protein, water, oil, and mucus," Dr. Kanakamedala wrote on the clinic's blog.

Gravity also plays a role in tear production when you lie down, according to Healthline. When you're sitting or standing upright, your tear drainage system can let extra tears pass through your tear ducts. But when you're horizontal, that extra water can't get to your tear ducts, instead building up in your eye. These natural responses to being sleepy plus the pressure that yawning puts on your lacrimal glands speed up tear production, making your eyes extra watery.